The present invention generally relates to magneto-optic recording mediums, and more particularly to a magneto-optic recording medium which is suited for the so-called mark edge recording system.
When recording information on a magneto-optic recording medium, marks of inverted magnetic domains are formed on the magneto-optic recording medium by use of a recording laser beam and an external magnetic field. In addition, when reproducing information from the magneto-optic recording medium, the recorded information is read with the use of a deflected reproducing laser beam and utilizes the magneto-optic effect.
Recently, due to the demands to improve the information recording density of the magneto-optic recording mediums, there have been demands to use a semiconductor laser which outputs a laser beam having a short wavelength as a light source of the recording laser beam and to employ the mark edge recording system.
Presently, the wavelength of the laser beam used for recording information on a magneto-optic disk is 780 nm or 830 nm. If the wavelength of the recording laser beam is set to the short wavelength, it is possible to make the recording marks formed on the magneto-optic disk small and accordingly, improve the recording density of the magneto-optic disk. Hence, a high density recording can be used with a laser beam which has a wavelength of approximately 680 nm and is about to be reduced to practice. However, the magneto-optic disk which is reduced to practice at present is made on the precondition that the recording laser beam used has a wavelength in a vicinity of 800 nm. For this reason, even if the wavelength of the recording laser beam is simply reduced to 680 nm, for example, there were problems in that the magneto-optic effect obtained will become small and that the quality of signals recorded on the magneto-optic disk will deteriorate.
According to the mark edge recording system, the edge of the recording mark formed on the magneto-optic disk corresponds to information (data) value "1". Hence, compared to the conventional so-called mark position recording system according to which the recording mark itself formed on the magneto-optic disk corresponds to the data value "1", the mark edge recording system can record the information on the magneto-optic disk with a higher density. It is conceivable to apply the mark edge recording system to the existing magneto-optic disk. However, no such magneto-optic disk has been reduced to practice at the present because of the need to realize a magneto-optic disk with reduced jitter and edge shift. The "jitter" refers to a positional deviation in the output waveform reproduced from the magneto-optic disk due to noise such as a medium noise of the magneto-optic disk. On the other hand, the "edge shift" refers to a positional deviation of the mark edge corresponding to the data value "1".
Various methods of improving the quality of signals recorded on the recording medium have been proposed, including the methods proposed in Japanese Laid-Open Patent Applications No. 57-78652 and No. 1-237945.
A thermomagnetic recording medium proposed in the Japanese Laid-Open Patent Application No. 57-78652 is provided with a high-coercivity layer having a low Curie point and a low-coercivity layer having a high Curie point. These two layers are commutatively coupled. The signal quality is improved according to this thermomagnetic recording medium.
On the other hand, a magneto-optic recording medium proposed in the Japanese Laid-Open Patent Application No. 1-237945 is provided with a high-coercivity layer made of a rare earth iron group amorphous alloy, and a low-coercivity layer made of a rare earth iron group alloy including light rare earth materials. These two layers are commutatively coupled. The signal quality is improved for the short wavelength according to this magneto-optic recording medium.
However, according to the methods proposed in the Japanese Laid-Open Patent Applications No. 57-78652 and No. 1-237945, a commutatively coupled layer structure is employed and an interface magnetic domain wall is generated between the layers. For this reason, differences are introduced between the sizes of the marks recorded in the two layers, and the positional accuracy of the marks which is essential in the case of the mark edge recording system is lost. As a result, the jitter and the edge shift are large according to these proposed methods, and there is a problem in that these proposed methods are unsuited for carrying out the mark edge recording using the short wavelength.